Eliciting and tracking evoked potentials during surgery is an established method for monitoring for potential nerve injuries. For example, electrically stimulating a patient during surgery and monitoring the resultant somatosensory evoked potentials (SSEPs) using conventional intraoperative neurophysiologic monitoring (IONM) systems is an accepted and useful clinical procedure that can identify changes in brain, spinal cord, and peripheral nerve function. Conventional IONM systems are typically used when the risk of severe nerve damage is relatively high, such as, for example, during brain and spinal surgeries. Early and accurate identification of changes in nervous system functioning may minimize the occurrence of long-term damage to structures of the nervous system.
Similarly, improved neurophysiologic monitoring devices and methods have been developed, which can be used to stimulate and monitor a patient's evoked potentials during other surgeries in order to identify and prevent positioning effect injuries. Such devices and methods are described in U.S. Pat. No. 8,731,654 to Johnson et al., the disclosure of which is herein incorporated by reference in its entirety. Positioning effect injury is an injury caused by undue tension or pressure on peripheral nervous structures. It can be caused by the position in which a patient is placed during surgery. Warning signs of positioning effect may include sensations, such as, for example, numbness, tingling, or weakness in a portion of the body. During surgery, a patient is typically placed under general anesthesia and unable to identify or react to the usual warning signs of positioning effect. Consequentially, patients may be left in compromised positions for the duration of a surgical procedure. Continued trauma from positioning effect may result in prolonged or even permanent injury to one or more peripheral nerves.
Intraoperative neurophysiologic monitoring is generally performed with a specialized computing device that delivers electrical stimulations to a patient's body and records signals produced by the body in response. Alternatively, spontaneously arising signals that do not require stimulation may be recorded. The specialized computing device typically performs some processing of the recorded signal(s), and healthcare professionals may monitor the processed signal for changes.
In order for monitoring to be effective noise and interference should be minimized. Reducing noise and interference is a particular concern when the target signals are very small such as with evoked potentials, because even the presence of a little noise can dramatically reduce the signal-to-noise ratio due to the small size of evoked potentials. Evoked potentials, such as, for example, SSEPs, are small bioelectric signals with amplitudes as small as one microvolt or less.
Techniques have been developed to reduce random noise present in processed biosignals. Unfortunately, current techniques are not sufficient. Interference remaining in the processed signal when using current techniques can significantly distort the processed signal. Accordingly, there is a need for improved signal acquisition and/or processing systems and techniques capable of further reducing or eliminating interference in the processed signal.